Reactor Theory (Neutron Characteristics)DOE-HDBK-1019/1-93NEUTRON FLUX SPECTRUMRev. 0Page 37NP-02Neutron Flux Spectrum SummaryPrompt neutrons are born at energies between 0.1 MeV and 10 MeV. Theaverage prompt neutron energy is about 2 MeV. Fast reactors have a neutron energy spectrum that has the same shape as theprompt neutron energy spectrum.Thermal reactors have a neutron energy spectrum that has two pronouncedpeaks, one in the thermal energy region where the neutrons are in thermalequilibrium with the core materials and another in the fast region at energieswhere neutrons are produced. The flux in the intermediate region (1 eV to0.1 MeV) has a roughly 1/E dependence.The neutron flux spectrum for the fast energy region of a thermal reactor has ashape similar to that of the spectrum of neutrons emitted by the fission process.The reason for the 1/E flux dependence at intermediate energy levels in athermal reactor is due to the neutrons' tendency to lose a constant fraction ofenergy per collision. Since the neutrons lose a greater amount at the higherenergies, the neutrons tend to "pile up" at lower energies where they lose lessenergy per collision.The neutron flux spectrum for the slow region of a thermal reactor contains apeak at the energy where the neutrons are in thermal equilibrium with the atomsof the surrounding material.SummaryThe important information in this chapter is summarized below.